In recent years, complementary metal-oxide semiconductor (CMOS) imagers have been widely used in digital still cameras, camcorders, surveillance cameras, and the like, and also the market thereof has increasingly expanded. In such a CMOS imager, pixels convert incident light into electrons using photodiodes. Then, the pixels accumulate the electrons for a given period, and then output signals reflecting the amount of accumulated electric charge to an analog-to-digital (A/D) converter built into a chip via amplifier elements disposed in the pixels. The A/D converter digitizes the signal from the pixels, and then outputs the digitized signal to an image processing circuit of a signal processing circuit at the following stage. In the CMOS imager, such pixels are disposed in a matrix shape for imaging (refer to, for example, Patent Literature 1).
To read a signal from such a pixel, the CMOS imager performs the following operation. That is, before accumulated electric charge is transferred to an input node (a floating diffusion layer, etc.) of an amplifier element within the pixel, the CMOS imager resets the input to, for example, a voltage of a power supply to make it float. At this time, reset noise (kTC noise) occurs in the input node of the amplifier, and thus the CMOS imager reads a reset signal of this reset state from the pixel, and performs sampling on the result to set it as a reference signal. Subsequently, the CMOS imager transfers the accumulated electric charge, reads a signal thereof from the pixel as an exposure signal, and then performs sampling thereon. Then, the CMOS imager takes the difference between the sampled signals, and offsets the kTC noise. This signal processing is called correlated double sampling (CDS).
Here, the signal output from each pixel through sampling includes random noise of the amplifier element disposed in the pixel or random noise of a detection circuit itself. As a method of reducing such random noise, a method of performing sampling a plurality of times on the reset signal and the exposure signal in the correlated double sampling and adding or averaging the results is effective. In this manner, performing sampling on the reset signal or the exposure signal a plurality of times in correlated double sampling will be hereinafter referred to as multiple sampling.